A. Field of the Invention
This invention relates to a firing control device and more particularly to an intervalometer for use with airborne rocket launchers.
B. Prior Art
In airborne armament, an intervalometer is a firing control device which successively delivers pulses of current to each one in turn of a plurality of electrically detonated rockets carried on board a tactical combat aircraft such as that shown in U.S. Pat. No. 3,396,628. In response to closure of a fire button by the pilot, a movable electrical contact in the intervalometer is sequentially positioned adjacent to contacts coupled to the individual rockets. In prior intervalometers, the duration of the current pulse delivered to a fuse of each rocket is dependent upon the solenoid characteristics. In the ripple, or repetitive, firing mode, the interval between the firing of one rocket and the next is dependent on these characteristics as well.
Prior intervalometers such as that shown in U.S. Pat. No. 3,539,955 maintain electrical grounding on each rocket fuse terminal until the moment the rocket is fired and whose continued operation is not interrupted by the presence of a short-circuited firing fuse. A movable electrical contact is rotated by a solenoid actuated pawl bearing upon a ratchet wheel. Auxiliary contacts operated by the solenoid motion first interrupt the current to the solenoid and secondly forwardly firing current to the movable contact. Since the duration of the solenoid current pulse is thereby controlled by the speed with which the solenoid responds, the amount of current built up in the coil is a variable and this in turn affects the speed with which the solenoid returns to the starting position to begin another advance of the contact. Since the current to the rocket fuse is available only during the time that the solenoid is pulled away from its rest position, the duration of the firing pulse is also heavily dependent upon the solenoid characteristics.
Accordingly, the duration of the current pulse to prior solenoids must remain within specified limits despite variations in manufacturing procedures and a wide range of temperature and humidity conditions to which the intervalometer is exposed during flight. Moreover, even the viscosity of the lubricant used in the solenoid and switch has an important effect and this can vary with age as well as temperature. If this duration is too brief, the duration of the pulse of firing current to the rocket may not be long enough to ensure reliable ignition of the propellant. If, on the other hand, the duration of the solenoid current pulse is excessive, the length of the interval between successive rocket launchings in the ripple mode may become too large and thereby cause too few of the rockets in the barrage to land on a small target area. The prior art left much to be desired in intervalometers which on one hand could be inexpensively produced but on the other hand would possess more consistent timing characteristics.
Accordingly, an object of the present invention is an intervalometer wherein the duration of the solenoid current pulse is fixed and independent of solenoid characteristics. Another object of the present invention is an intervalometer wherein the interval between successive rocket firings in the ripple mode is similarly fixed. Another object of the present invention is the elimination of moving contacts in the circuits which control solenoid current. Another object of the present invention is generation of firing pulses whose duration is not affected by solenoid operation.